Heat treatment method for ferrite-pearlite steel

ABSTRACT

THE INVENTION RELATES TO A HOT FORMING METHOD FOR PRODUCING STEEL WITH A FERRITE-PEARLITE STRUCTURE. THE STEEL OF THE INVENTION HAS IMPROVED TOUGHNESS.

United States Patent 1m. (:1. czid 7/14 US. Cl. 148l2 3 Claims ABSTRACTOF THE DISCLOSURE The invention relates to a hot forming method forproducmg steel with a ferrite-pearlite structure. The steel of theinvention has improved toughness.

BACKGROUND OF THE INVENTION The present invention relates to a heattreatment method for improving the toughness of ferrite-pearlite steels.

DESCRIPTION OF PRIOR ART In the prior art, many and varied methods havebeen proposed for improving the toughness of ferrite-pearlite steels,such as adjusting the hot forming conditions or using a normalizingtreatment.

There are many objections to making adjustments to the heatingconditions for the steel during the hot forming process, e.g. processingtemperature, or time of processing. With lower processing temperature,the deformation resistance and work hardening rate of the steelsincrease thus resulting in lower working efiiciency. Moreover, it isvery diflicult to carry out industrially the required processing becauseof the limited processing capacities of rolling machines. A normalizingtreatment is quite a good heat treatment method, although varioustechnical and financial short-comings having still to be removed. In anormalizing treatment, a steel after being processed, is cooled down toa normal temperature and then reheated to an austenitic temperature.Thus, not only is the quantity of heat used very high, but also muchspace is required to cool the steel. Also, the time lag between hotforming and the heat treatment is liable to lower the workingefiiciency. Finally, the prolonged heating produces scales on thesurface of the steel during the heating stage which cause the surfacequality of the steel to deteriorate.

SUMMARY OF THE INVENTION The object of the present invention is toovercome various defects of the known methods. More particularly,according to the present invention there is provided a method forimproving the toughness of steels having a ferrite-pearlite structure inwhich the steel, after hot forming, is at once cooled down to 700-500 C.to transform the austenitic structure into the ferrite-pearlitestructure by about 5090%, and then again is heated to above the A3transformation point for austenitization, and cooled by air or by someother suitable cooling methods.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The treatment of the presentinvention can be applied "ice to any steels with a ferrite-pearlitestructure, so that there is no particular need to use a steel with thestandard range of components. However, when there are too many alloyelements involved, the transformation point is lowered and theapplication of this treatment difficult. Accordingly, in practice, itwill be generally desirable to fix the range of components, by weight,as follows:

Carbon: The upper limit of C content is fixed at 0.25%. As thepercentage of carbon increases, the toughness is reduced and thetransformation point is lowered so making the application of thistreatment more diiiicult. The lower limit is fixed at 0.05% due to thelimitations of industrial steel making for structural steels whichrequire great toughness.

Si: The upper limit of Si content is 1.00%. Si is very effective as adeoxidizer in steel making and makes the treatment of the inventioneasier as it serves to raise the starting temperature of ferritetransformation. On the other hand, more than 1.00% of Si will lower thetoughness of the steel. Therefore, the upper limit is fixed at 1.00%.

Mn: Mn is a cheap but very effective element for improvin the toughnessof steels. However, a higher percentage of Mn content causes thetransformation point to go down and hinders the successful applicationof the treatment of the invention. The upper limit is fixed at 1.50%.The lower limit is fixed at 0.40% which is generally regarded as anessential percentage of Mn content as a deoxidizer in steel making.

The percentages of other elements are set out as follows:

Less than 0.50% for Ni, Cr, Cu; less than 0.30% for Mo,

W; less than 0.15% for V, Nb, Ti, Zr, Al. These elements, if they exceedthe mentioned percentages, will greatly lower the transformation points.This is not de sirable in this treatment, as it reduces the toughness ofthe steel.

After hot forming, the maximum for the intermediate cooling temperatureis limited to 700 C. A temperature exceeding this maximum results in theslowing of the transformation from austenitic to ferrite-pearlitestructure, hence lowering the working efiiciency. (Refer to treatment 2in Table l.)

The minimum cooling temperature is set at 500 C. Below this temperature,the loss of heat retained in the steel will be greater. Moreover, thesubsequent heating will be prolonged so increasing the formation ofscales.

Within this temperature range, the amount of transformation fromaustenitic to a ferrite-pearlite structure after hot rolling is about50-90%. If it is less than 50%, the treatment will be less effective asthe after effect of hot rolling is still very pronounced, while if it ismore than the transformation speed will be slower, so that the treatmenttime will be prolonged and lose its industrial advantages.

It structures other than ferrite-pearlite increase in the finallyproduced steels, the toughness of the steels will be lowered.Accordingly, the maximum is fixed at 90%.

In additiomafter A3 transformation point, steels could Example The tableshows the comparison of characteristics obtained by various treatmentsof a 20 mm. thick steel Thus the present invention provides manytechnical and economic advantages in improving the toughness of steels.

We claim:

1. A hot forming method for producing a steel consisting essentially of0.050.25% of C, less than 1.00% of Si, 0.40-1.50% of Mn, and one or moreof the following elements in the stated percentages: less than 0.50% ofNi, Cr and Cu, less than 0.30% of Mo and W, and less than 0.15% of Nb,V, Ti, Zr and A1, with the resheet containing 0.14% of C, 0.37% of Si,1.26% of Mn, mainder essentially Fe with -a ferrite-pearlite structure,

0.012% of P, 0.010% of S and 0.05% of Al.

TABLE 1 Tensile test 2 mm. V notch Austenitic (1 IS No. 4 steel) impacttest transformation amount Thick- Yield, Tensile Elongain intermedinessof oint, strength, tion rate, E0, Trs, Tr 15, ate cooling, scale, kg.mm. kgJmm. percent kgJm. 0. C. percent mm.

1. Air cooling after hot forming (as formed) 33. 2 51. 1 88. 8 8. 4 -10-73 0. 10 2. Cooled down to 700 C. after hot forming, heated at I 900 C.for min. andthen cooled by air 35. 4 51. 2 38. 1 9. 4 ---10 74 5 0. 173. Cooled down to 650 C. after hot forming, heated at 900 C. for 15 min.and cooled by air 33. 4 50. 8 40. 5 17. 4 56 78 65 0. 18 4. Cooled downto 550 C. after hot forming, heated at 900 C. for 15 min. and cooled byair 38. 2 51. 0 40. 5 18. 4 -61 99 90 0. 19 5. Cooled down to normaltemperature after hot forming,

heated at 900 C. for 15 min. and cooled by air ,(normaiizing)- 36. 3 51.9 38. 8 20. 0 68 -100 100 0.

In the table, treatment shows thestate of a steel as hot rolled andtreatment 5 is the conventional normalizing treatment. Treatment 3 and 4embody the present invention and clearly indicate the great improvementof toughness in comparison with other treatments.

More particularly, in treatment 3, the steel is cooled down to 650 C.after hot forming. The austenitic transformation obtained is 65% whichis a great improvement over the treatment 1. It shows almost the samevalue of absorbing energy E, and a 50% brittle fracture percentage ,Trsas produced by the conventional normalizing treatment 5. Further, in thetreatment 4 in which a steel is cooled down to 550 C., the 15 ft.-1b.transition temperature Tr 15 is almost equal to that of the conventionaltreatment 5.

As will be seen from the above description the present inventionprovides a method for producing steels with various advantages over theprior art; the quantity of heat used can be reduced as the heat retainedby a steel after hot forming can be effectively utilized in reheating;operating efiiciency of a hot treatment furnace can be improved bydrastically shortening the heat treatment time, and the workingefficiency can be improved by reducing the space for transferring steelsand shortening the time lag between forming and heat treatment. Anotheradvantage, as will be apparent from the table, is that the shortening ofthe heating time serves to reduce the scale formation on the surface ofthe steels.

comprising the steps of hot forming the steel with an austeniticstructure, air cooling the formed steel to a temperature within therange of 700 to 500 C. and sufiicient to transform 50%-90% of theaustenitic structure to the ferrite-pearlite structure, heating thetseel to a temperature above the A3 transformation point foraustenification and air cooling the steel to obtain a product havingsubstantially 90% of ferrite-pearlite structure.

2. The method of claim 1, wherein said hot formed steel with anaustenitic structure, is air cooled to 650 C.

3. The method of claim 1, wherein said hot formed steel with anaustenitic structure, is air cooled to 550 C.

References Cited UNITED STATES PATENTS 3,432,368 3/1969 Nakamura 1481342,858,206 10/1958 Boyce et a1. 148-12 3,010,822 11/1961 Altenburger eta1. -123 3,102,831 9/1963 Tisdale 148-l2 3,163,565 12/1964 Wada 148-1433,201,288 8/1967 Grange 148l2.4 3,388,988 6/1968 Nagashima et a1.148--12 3,492,173 1/ 1970 Goodenow 14812 WAYLAND W. STALLARD, PrimaryExaminer US. Cl. X.R. 148134 y 3 s R; t "1v n w 6 m t d t a... n n a Mm4 m a K P M n w m u M 9 w t t H 0 N 1 d o n C n v e l S i S T m .3 1 e io 3 T e w l v b. P W Wm H 3 v {V mm w 1 6.1 3 r e o i d s M 4 e m m h lV/ T S a PM b W e a Ri o e S .1 M E m w t v a n Y B m W WHHM C w d 2 H M0 O R O 1 v RC PU f & .m 3 g m a 8 m W m t r I d d t n w e a .w M n C te eh m 3 VJ 1 9 n n rb e r 2 T. .H? a e d e PU e r d s S 1 WM n .1 W 1 aa 1 u h PM I m 9 ,t a L o W mi a m a M m2 d g ANA r t b 3 e m e n a l le T O a R u J1 U .E t C 6 Wm a a J W; s h P 3 t S 9 1 O t 3 e r R T T V14 s h e d E e MAL m y OJ I d r n t S n H C W mm 6 M .mm. m n a Ci mwl qU PI t n Rm 1 S i e 1 1 d E 1 O T t L o e e 7 A r C e C n P T w m u h wg A Q s M m m P 5 Mn an O (x 1 t V S )t D .t M N u t a t t L w m MW w tt I m. s i A t W t o n n r E t D .t 4 e e l n S n 0 t V (A E P a n n f lM P I a R o F

